1. Field of the Invention
The present invention relates to a phosphor of a type suitable for use in a detector for high-energy radiation, such as x-rays.
2. Description of the Prior Art
Detectors can be constructed of a phosphor and a photodiode or, respectively, a photomultiplier for detecting high-energy radiation. Such detectors are widely employed in nuclear medicine and in x-ray diagnostics. The function of the phosphor is to absorb the high-energy radiation and to emit visible light as a consequence of this absorption. This visible light can be detected by a photosensitive element, for example a photodiode, a photomultiplier or a light-sensitive film.
In modern radiation detectors as employed, for example, in x-ray computed tomography, phosphors having only an extremely slight afterglow are required in order to achieve an adequately high read-out frequency. A phosphor in widespread use is thallium-doped cesium iodide (CsI:T1), but this phosphor still exhibits an afterglow intensity of approximately 10.sup.-2 through 10.sup.-3 of the initial light intensity, at a time, for example, of 20 msec after the high-energy irradiation of the phosphor has ceased. Phosphors whose afterglow has decayed to less than 10.sup.-4 of the initial intensity after approximately 5 through 10 msec, however, are required for state of the art radiation detectors.
The oxisulfides of the rare earths are considered to be promising phosphors for employment in modern radiation detectors. German OS 36 29 180 discloses a method for manufacturing a phosphor ceramic having the general composition (Ln.sub.1-x-y M.sub.x Ce.sub.y).sub.2 O.sub.2 S:X with Ln=Gd, La or Y; M=Eu, Pr or Tb and X=F or Cl with 0&lt;(x,y)&lt;1. The pigment powder employed as initial material is thereby filled into a vacuum-tight metal container and is compressed to form a ceramic by isostatic hot-pressing.
A modified method is disclosed in U.S. Pat. No. 5,296,163 corresponding to German Patent Application P 42 24 931.7. Therein, a rare earth sulfite is precipitated from solution and is subsequently reduced to form oxisulfide. The pigment powder obtained in this way has a high specific surface of more than 10 m.sup.2 /g (according to BET) and can be compressed to form a translucent phosphor ceramic having high density and good phosphorescent properties on the basis of a simple, single-axis hot-pressing process.
A disadvantage of this method is the high reactivity of the pigment powder caused by the high specific surface thereof and the high reactivity of the as yet incompletely compressed phosphor ceramic, so that a surface oxidation to form rare earth oxisulfate is virtually impossible to prevent as a practical matter given technical handling of the pigment powder. The presence of surface oxidation prevents the formation of an optimum microstructure during hot-pressing and leads to the creation of secondary phases of oxisulfate that in turn reduce the luminescent intensity.